[Gmecm] TBI Swap - Iron Duke Tech IV

[Darryl M Gilbert]

Hello: Just my 2 cents worth. I have a spare cap that i hoged out all the
plastic cap material from around no# 1 tower so I could look into the cap
and see what the spark is actually doing. It works quiet well to actually
see what is happening. When sun is gone it makes quiet a show and is very
interesting to see how it all works as RPM goes up and down. Can also
check just how well the rotor to tower phasing is. Another thing is can
use the timing light to look into thehole in the cap and see how it is
all working. Be sure if you have an adjustable light you do not
accidently bump the knob off 0 to give false indications ( ask me how I
know Ha!)
good luck
Darryl  (then there is always CIS injection)

On Sat, 18 Feb 2006 08:14:02 -0800 (PST) Andrew Gibson
<andrewsharyn at yahoo.com> writes:
>> the rotor may not be close enough to the tower to fire correctly.

>In regards to the rotor contact area, it is better to actually observe
the
>rotor of a given application, as I've seen rotors with contact terminals
>15/16" wide.  

Some are 15/16" wide. But the rotors used on TBI engines are about 1/4"
wide. So if it's not close enough to the tower, you have a wide gap,
which means more voltage required to jump the gap and less burn time at
the plug. AKA-misfire

>> One other issue I want to share, is the injector timing. It's not
enough 
>> just to fire an injector when the spark fires. The injector MUST fire 
>> when the air is just beginning to be sucked into a cylinder....

>> The window to get the fuel in decreases as the RPM go up. So if the 
>> injector timing is off you may miss that window, and get a misfire 
>> from poor fuel atomization. 

>You're making this out to be more technical and precise than it really 
>is.  After ~4000 RPM it is common for SEFI to become batch fire.
>While not 100% DC, this does imply spraying fuel to the back of a 
>closed valve.  This explains why there is virtually no HP difference
>from batch fire to sequential, or bank to bank, FTM.  Sequential's
>strong suit is lower part throttle emissions and improved fuel economy.

Sequential injection does have better emissions. Why? Because more fuel
is burned at lower rpms due to timing of the injectors. If you throw off
the timing it is common to lose 35-50 HP. A good example is if a Ford (I
know this is the gm section) wipes out it's cam sensor. The injectors
switch to batch fire and my clients usually complain of a check engine
light and a gutless car. Another good example is the Mazda 2.6L 4cyl. The
distributor can be put in at 3 different positions. Even though it will
run at all three positions, only one will work right. If wrong the gas
milage drops to about 6 mpg, and the truck is flat out gutless. All
becasue the synch pulse for the injectors is in the wrong place. I made
that mistake once and the owner was quite upset.

As for batch firing above 4000rpm: There is not enough time to fire a
injector with the precision you can below that speed. And besides, with
the air velocity you have at that speed, atomization of the fuel is not
an issue.

>> ... which is usually after the spark event. 

>:scratching head:

At least you arre thinking. Ignition timing is typically running between
6 to 32 degrees BTDC. The intake valve does not start to open until 1 to
2 degrees BTDC. So if you fire the injector too early air is just
compressing behind the valve, not moving. So you miss the rush of air
that will mix the fuel in well.

One other thing you may be interested to know. GM TBI fires the injectors
when the ecm recieves the signal pulse from the distributor or ignition
module. However I'm sure you know that EST come into play. But only for
the spark. The ecm sends a signal back to the module to modify the
ignition timing, without modifying the injector timing. When is the
injector fired? At 0 Degrees BTDC. Right where you set the timing.


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